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K. W. BEWIGAND W. A. ZISNAN
Vol. 67
PST~ESTIGATION OF SOLUTION ADSORPTION ON PLATINUM OF PURE AND JI1ICIXED FILMS OF FATTY AMINES BY CONTACT POTENTIALS1 BY K. W. BEWIGAND W. A. ZISMAN U.S. Naval Research Laboratory, Washington 25, D. C. Received J u n e 20, 1962 A homologous series of pure, primary, fatty amines from Cd to Czzhas been studied in the form of retracted monomolecular films adsorbed on pure polished platinum. The change in the contact potential difference between the platinum electrode and a stable reference electrode of gold coated with FEP Teflon as the result of the adsorption of each condensed monolayer was measured a t 20” and 50y0 relative humidity in filtered air carefully freed from organic contaminants. The molecular packing of amine molecules in each monolayer was measured conveniently although indirectly by obtaining the contact angle of pure methylene iodide as in our previous investigations. Contact potential differences were reproduced readily under these conditions in independent measurements within 1 2 % provided the electrode temperature and the relative humidity were controlled appropriately. Monolayers were deposited on the platinum electrode by retraction from a variety of pure solvents. Plots of the contact angle us. the number of carbon atoms per amine molecule and of the contact potential differences vs. the contact angle were found most informative about the effects of homology, molecular packing, and orientation, as well as of solvent inclusion through molecular adlineation in the monolayers. The molecular adlineation of solvents containing a phenyl group as well as branching of the hydrocarbon chain were compared with that of straight chain alkane solvents.
Introduction a f t e r a metal surface has been coated with a film of either Teflon or FEP Teflon, there is practically no tendency to adsorb gases or vapors which are above their boiling points.2 An unreactive metal, like gold, when so coated is an effective reference electrode for studying the adsorption of molecules on an uncoated or “active” metal electrode by means of measurements of change in the contact potential difference between the electrodes. This useful surface property of fluorocarbon resins follows from their extraordinarily low surface energies (or low critical surface tensions of wetting). For example, the critical surface tension of wetting of Teflon is 18.5 dynes/cm.,a and of FEP Teflon only 16.2 d y n e ~ / c m . ~Recently, Martinet5 and Grahama have shown that Teflon has low gas adsorptivity a t ordinary temperatures. We report here the changes observed in the contact potential difference between such a reference electrode and the clean polished surface of platinum (the active electrode) upon which a close-packed monolayer of each of the homologous fatty amines has been adsorbed. Results are also presented on the effects of varying the solvent from which the amine is adsorbed, the time allowed for adsorption, the conditions under which mixed condensed films of solute and solvent are adsorbed, and on the relation between structure of amine and solvent molecules for stable mixed films to form. Experimental Materials and Procedures.-The primary fatty amine compounds used as adsorbates in this investigation were of unusual purity; when each was melted and recrystallized in flowing dry nitrogen gas, a melting point range within 1 or 2’ of the best literature values was obtained. All of the contact potential measurements were made using the vibrating condenser method7r8 in a room ventilated with filtered air freed of adsorbable vapors and controlled to 50 f 57, relative humidity and 20 i 1’.
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(1) Presented a t the National Meeting, Division of Colloid and Surface Chemistry, American Chemical Society, Washington, D. C., March 29, 1962. (2) (a) K. W. Bewig and W. A. Zisman, NRL Report 5383, “Metals Coated with Films of Low Surface Energy as Reference Electrodes for the Measurements of Contact Potential Differences,” October 23, 1959; (b) X. W. Bewig and W. A. Zisman, Advances in Chemistry Series, No. 33, American Chemical Society, Washington. D. C., 1961, p. 100. (3) H. W. Fox and W. A. Zisman, J . Colloid Sei., 6, 514 (1960). (4) M. K. Bernett and W. A. Zisman, J . Phgls. Chem., 66, 2266 (1961). (5) J. M. Martinet, Rapport CEA 888, Centre D’Etudes Wucleaises do Socloy, 1958. (6) D. Graham, J . Phiis. Chem., 08, 1815 (1962).
The platinum surface on which each monoIayer was adsorbed was prepared by polishing it on a wet “kitten’s ear” cloth with levigated alumina having an average particle size of 0.3 ,A; it then wm scrubbed on a clean, grease-free cloth, rinsed in flowing distilled water, and dried in clean air. I n the carefully controlled air used, this treatment gave adequately reproducible contact potential differences between the reference electrode and the clean platinum electrode and also the platinum electrode after having been coated with an adsorbed monolayer of the amine. Reproducibility was always within f10 mv. even after 24 hr. When platinum was cleaned in this way, much more reproducible data were obtained than after flaming, or after various etching treatments, or after cleaning with redistilled solvents. Of course, none of these treatments in the air produced platinum surfaces free of oxide. Each of the fatty amines studied was adsorbed on the platinum by retractiong-I1 from a solution in pure nitromethane or in each of a variety of pure hydrocarbons. The contact potential difference referred to throughout this report was obtained by first allowing a freshly cleaned, plane, platinum electrode which was S/S in. in diameter and l/ls in. thick to come to equilibrium with the room atmosphere for 30 min., a t which time the difference in contact potential between it and the coated reference electrode was measured. Next, the electrode was removed from the vibrating condenser and placed in a solution of the amine for the desired immersion time. Finally, the electrode was removed slowly from the solution with its plane face held vertically in order to allow the liquid to retract, and the resulting dry, monolayer-covered electrode was inserted in the vibrating condenser. The resulting potential difference was found to be constant for a long time and so could be measured readily. The difference in the contact potential before and after the platinum had been so immersed and retracted from the adsorbing solution will be denoted as A V . The contact angle (e) reported here is the value measured with a sessile drop of pure, colorless, methylene iodide (surface tension, 50.8 dynes/cm. at 20’) resting on the film-coated, horizontal, platinum surface of the active electrode. A contact angle of from 68 to 70” always is obtained with methylene iodide when resting under equilibrium conditions on the close-packed methyl terminal groups of a film comprised of paraffinic polar molecules.11-13 We have used methylene iodide extensively for measurements of contact angles on films because its high surface tension leads to large values of 6, the approximate spherical shape of the molecule minimizes any tendency to molecular ad(7) W. A. Zisman, Rev. Sei.Instr., 3, 369 (1932). (8) K. Bewig, NRL Report 5096, “Improvements in the Vibrating Condenser Method of Measuring Contaot Potential Differences,” February 4 , 1958. (9) W. C. Bigelow, D. L. Pickett, and W. -4.Zisman, J . Colloid Sci., 1, 513 (1946). (10) W. C. Bigelow, E. G. Glass, and A ’ .’ A. Zisman, ibid., 2, 563 (1947). (11) E. G. Shafrin and W. A . Zisman, J . Phys. Chem., 64, 519 (1060). (12) 0. Levine and W. A. Zisman, ibzd., 61, 1068 (1957). (13) 0. Levine and W. A. Zisman, ibid., 61, I188 (1957).
Jan., 1'963
SOLUTION ADSORPTION ON PLATINUM OF PURE AND MIXEDFILMS OF FATTY AMINES
lineation in films of polar paraffinic compounds, and the large size of the methylene iodide molecule prevents liquid or vapor permeation into close-packed monolayers of paraffinic derivatives. The alkane solvents used in these experiments were 99+% olefin-free; they were obtained from Phillips Petroleum C o ~ ,, p Humphrey-Wilkinson, and Matheson, Coleman and Bell, and before each sample was used it was percolated through a column of activated silica gel and adsorption alumina. An exception was n-octadecane, which has a melting point of 28"; this compound was liquefied a t 35" and i t was used as obtained from the supplier. The nitromethane solvent was Eastman spectro grade and it waa also used without further purification.
Results with Adsorbed Films which Are Free of Solvents.-Figure 1 is a plot of the values of AV obtained for the homologous series of primary fatty amines when plotted against the number ( N ) of carbon atoms in the amine molecule. Each monolayer was adsorbed from a 0.10 wt. % solution in 10 ml. of nitromethame. Previous research9-13had shown that this concentration was more than sufficient to guarantee adsorption of each amine on platinum in the form of a close-packed monolayer. Nitromethane has a surface tension a t 20" of 36.2 dynes/cm., which is sufficiently greater than the critical surface tension of wetting of 24 dynes/cm. observed with close-packed paraffinic films to permit isolating the platinum coated with its adsorbed moriolayer from the solution by the retraction method.11-13 Each graphical point in Fig. 1was unchanged when the time of immersion before retraction was varied from 12 min. up to 24 hr.; consequently, with the 0.1% concentration of amine used, the equilibrium adsorbed film was always obtained within the first few minutes of iimmersiori of the platinum. However, to be certain that the time of adsorption would not be a factor in plotting Fig. 1 for every member of the homologous series of amines, an immersion time of 30 min. was always used. The reset accuracy of any one measurement of LrV was within 2 or 3 mv. ; the the spread of a series of inclependent experiments is shown in Fig. 1, the average deviation from the curve being A20 mv. The graph reveals an asymptotic maximum for tetradecylamine (c14) and its higher homologs. Because of the nonlinear shape of nitromethane and its volatility, mimed films of solute and solvent would not be expected a t adsorption equilibrium a t the high solute concentrations used; the following results will show that no evidence to the contrary was found. Hence, the observed values of AV must result from the change in the surface density of amine dipoles adsorbed on the platinum. Since there is no variation in the dipole moment of the higher fatty amines, the conclusion appears unavoidable that the number and orientation of the adsorbed amine molecules must be identical in all monolayers for which N 2 14. The simplest interpretation is that these filnis are close-packed monolayers. The progressive decrease in AV for the amines below c14 must mean that the decreasing lateral attractions by hydrocarbon "tails" having values of N below 14 permit either large, lateral, thermal movements in the chains or tilting (of the molecular axes in the adsorbed film. Therefore, the surface density of the adsorbed amine dipoles must decrease with N . The positive sign of the values of AV means that the adsorbed amine molecules were oriented in all cases so that the positive pole was directed away from the metal substratea2 A condensed monolayer of octadecylamine on an aqueous substrate a t pH 8.2 has a value of AV = 0.600
131 1
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I
I
I
I
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I
10 I2 14 16 18 20 N. NUMBER OF CARBON ATOMS IN MOLECULE OF AMINE,
Fig. 1.-Contact
8
LI
22
potentials of homologous amines adsorbed' on platinum from nitromethane.
v. and a limiting area per molecule of 20.4 B.2 a t zero c0mpression.1~ Using the Helmholtz relation, AV = 4nqpL, to compute pI, the normal component of the dipole moment of the molecules adsorbed on the aqueous substrate, where q = 1/(20.4 X 10-l6) or 4.9 X l O I 4 adsorbed molecules per cm.2, we obtain p I = 0.324 D. The literature value of the dipole moment of octadecylamine in the gaseous state is 1.3 D.15 The smaller value of pL of octadecylamine when adsorbed on water probably is the resultant of the induced dipoles caused by proximity of the amines and also the effects of dipoles in the surface layer of the liquid substrate which originate in hydrogen bonding and also in orientation in the local field of the amine dipoles. The situation of a condensed monolayer on a metal substrate is analogous to that of an aqueous substrate. Different metals have different work functions so that the interaction energy between a condensed monolayer and the underlying solid would be different for each metal; consequently, the resultant of the dipole polarization effects would change in each ca,se. A further complication arises from the fact that metals react differently with any one reference atmosphere so that their apparent work functions would differ from the values obtained from a nascent state surface. For these reasons, when the Helmholtz relation is used to compute p ~ the , apparent normal component of the dipole moment of the adsorbed molecules would be expected to change for different metal adsorbents and there would not necessarily be a one to one corresporidence with the work function sequence of the metals. I n any case, M L would differ from the dipole moment, p , of a molecule determined from dielectric constamt measurements of its vapor or of its solution in a nonpolar solvent. Using a multiple-dip technique, Bigelow, Pickett, and Zismangadsorbed octadecylamine on a platinum dipper from solution in dicyclohexyl and determined the average cross Fectional area per adsorbed molecule to be about 30 A.2 Brockway and Karle16and Bigelow and Brockwayl' have shown by electron diffraction experiments that there is a random tilt of several degrees in the axes of polar molecules adsorbed by retraction Ion solid substrates. Consistent with this is the structural model of micelles of such films proposed by Epstein.18 Either such a micellar structure or a slight random tilt of the adsorbed molecular axes could account for less (14) H. W. Fox, J . Phus. Chem., 61, 1058 (1957). (15) J. W. Smith, J . Chem. Soc., 1567 (1933). (16) L. 0. Brookway and J. Karle, J. Colloid Sci., 2, 277 (1947). (17) W. C. Bigelow and L. 0. Brockway, %bid., 11, 60 (1956). (18) € T. I. Epstein, J . Phys. Colloid Chem., 6 4 , 1053 (1950).
K. W. BEJVIGAXD W. A. ZISMAN
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